Method and apparatus for maintaining colour sequence when printing

ABSTRACT

An elongate print head with a longitudinal axis is provided, which comprises at least a first array of marking elements and a second array of marking elements for printing different colours. Each array of marking elements comprises a set of equally spaced marking elements. A first marking element of the first array of marking elements is spaced apart in the direction of the longitudinal axis from a first marking element of the second array of marking elements over a distance of at least n/P or n/I of a length of an array of marking elements, n being an integer larger than 1, P being a number of mutually interstitial printing steps used when printing an image and I being a number of interlacing steps used when printing an image.  
     A printer provided with such a print head is also provided.

[0001] The present invention relates to methods and apparatus forprinting, such as ink jet or thermal transfer printing, especiallynon-contact printing.

TECHNICAL BACKGROUND

[0002] Printing is one of the most popular ways of conveying informationto members of the general public. Digital printing using dot matrixprinters allows rapid printing of text and graphics stored on computingdevices such as personal computers. These printing methods allow rapidconversion of ideas and concepts to printed product at an economic pricewithout time consuming and specialised production of intermediateprinting plates such as lithographic plates. The development of digitalprinting methods has made printing an economic reality for the averageperson even in the home environment.

[0003] Conventional methods of dot matrix printing often involve the useof a printing head, e.g. an ink jet printing head, with a plurality ofmarking elements, e.g. ink jet nozzles. The marking elements transfer amarking material, e.g. ink or resin, from the printing head to aprinting medium, e.g. paper or plastic. The printing may be monochrome,e.g. black, or multi-coloured, e.g. full colour printing using a CMY(cyan, magenta, yellow, black=a process black made up of a combinationof C, M, Y), a CMYK (cyan, magenta, yellow, black), or a specialisedcolour scheme, (e.g. CMYK plus one or more additional spot orspecialised colours). To print a printing medium such as paper orplastic, the marking elements are used or “fired” in a specific orderwhile the printing medium is moved relative to the printing head. Eachtime a marking element is fired, marking material, e.g. ink, istransferred to the printing medium by a method depending on the printingtechnology used. Typically, in one form of printer, the head will bemoved relative to the printing medium to produce a so-called raster linewhich extends in a first direction, e.g. across a page. The firstdirection is sometimes called the “fast scan” direction. A raster linecomprises a series of dots delivered onto the printing medium by themarking elements of the printing head. The printing medium is moved,usually intermittently, in a second direction perpendicular to the firstdirection. The second direction is often called the slow scan direction.

[0004] The combination of printing raster lines and moving the printingmedium relative to the printing head results in a series of parallelraster lines which are usually closely spaced. Seen from a distance, thehuman eye perceives a complete image and does not resolve the image intoindividual dots provided these dots are close enough together. Closelyspaced dots of different colours are not distinguishable individuallybut give the impression of colours determined by the amount or intensityof the three colours cyan, magenta and yellow which have been applied.

[0005] In order to improve the veracity of printing, e.g. of a straightline, it is preferred if the distance between dots of the dot matrix issmall, that is the printing has a high resolution. Although it cannot besaid that high resolution always means good printing, it is true that aminimum resolution is necessary for high quality printing. A small dotspacing in the slow scan direction means a small distance between markerelements on the head, whereas regularly spaced dots at a small distancein the fast scan direction places constraints on the quality of thedrives used to move the printing head relative to the printing medium inthe fast scan direction.

[0006] Generally, there is a mechanism for positioning a marker elementin a proper location over the printing medium before it is fired.Usually, such a drive mechanism is controlled by a microprocessor, aprogrammable digital device such as a PAL, a PLA, a FPGA or similaralthough the skilled person will appreciate that anything controlled bysoftware can also be controlled by dedicated hardware and that softwareis only one implementation strategy.

[0007] One general problem of dot matrix printing is the formation ofartefacts caused by the digital nature of the image representation andthe use of equally spaced dots. Certain artefacts such as Moiré patternsmay be generated due to the fact that the printing attempts to portray acontinuous image by a matrix or pattern of (almost) equally spaced dots.One source of artefacts can be errors in the placing of dots caused by avariety of manufacturing defects such as the location of the markerelements in the head or systematic errors in the movement of theprinting head relative to the printing medium. In particular, if onemarking element is misplaced or its firing direction deviates from theintended direction, the resulting printing will show a defect which canrun throughout the printing. A variation in drop velocity will alsocause artefacts when the printing head is moving as time of flight ofthe drop will vary with variation in the velocity. Similarly, asystematic error in the way the printing medium is moved relative to theprinting medium may result in defects which may be visible. For example,slip between the drive for the printing medium and the printing mediumitself will introduce errors. In fact, any geometrical limitation of theprinting system can be a source of errors, e.g. the length of theprinting head, the spacing between marking elements, the indexingdistance of the printing medium relative to the head in the slow scandirection. Such errors may result in “banding” that is the distinctimpression that the printing has been applied in a series of bands. Theerrors involved can be very small—the colour discrimination, resolutionand pattern recognition of the human eye are so well developed that ittakes remarkably little for errors to become visible.

[0008] To alleviate some of these errors it is known to alternate orvary the use of marker elements so as to spread errors throughout theprinting so that at least some systematic errors will then be disguised.For example, one method often called “shingling” is known from U.S. Pat.No. 4,967,203 which describes an ink jet printer and method. Eachprinting location or “pixel” can be printed by four dots, one each forcyan, magenta, yellow and black. Adjacent pixels on a raster line arenot printed by the same nozzle in the printing head. Instead, everyother pixel is printed using the same nozzle. In the known system thepixels are printed in a checkerboard pattern, that is, as the headtraverses in the fast scan direction a nozzle is able to print at onlyevery other pixel location. Thus, any nozzle which prints consistentlyin error does not result in a line of pixels in the slow scan directioneach of which has the same error. However the result is that only 50% ofthe nozzles in the head can print at any one time. In fact, in practice,each nozzle prints at a location which deviates a certain amount fromthe correct position for this nozzle. The use of shingling candistribute these errors through the printing. It is generally acceptedthat shingling is an inefficient method of printing as not all thenozzles are used continuously and several passes are necessary.

[0009] As said above, this kind of printing has been called “shingling”.However, printing dictionaries refer to “shingling” as a method tocompensate for creep in book-making. The inventors are not aware of anyindustrially accepted term for the printing method wherein no adjacentpixels on a raster line are printed by one and the same nozzle.Therefore, from here on and in what follows, the terms “mutuallyinterstitial printing” or “interstitial mutually interspersed printing”are used. It is meant by these terms that an image to be printed issplit up in a set of sub-images, each sub-image comprising printed partsand spaces, and wherein at least a part of the spaces in one printedsub-image form a location for the printed parts of another sub-image,and vice versa.

[0010] Another method of printing is known as “interlacing”, e.g. asdescribed in U.S. Pat. No. 4,198,642. The purpose of this type ofprinting is to increase the resolution of the printing device. That is,although the spacing between nozzles on the printing head along the slowscan direction is a certain distance X, the distance between printeddots in the slow scan direction is less than this distance. The relativemovement between the printing medium and the printing head is indexed bya distance given by the distance X divided by an integer.

[0011] There is a continuous requirement for improvements in printingmethods and printers. In particular, there is a requirement to increasethe efficiency of printing using the minimum number of passes whileproviding high quality.

[0012] It is an object of the present invention to provide a printingmethod and apparatus which provides high resolution printing at highspeed with a reduced visible effect of systematic errors.

SUMMARY OF THE INVENTION

[0013] In one aspect of the present invention, an elongate print headwith a longitudinal axis is provided, which comprises at least a firstarray of marking elements and a second array of marking elements forprinting different colours. Each array of marking elements comprises aset of equally spaced marking elements. A first active marking elementof the first array of marking elements is spaced apart in the directionof the longitudinal axis from a first active marking element of thesecond array of marking elements over a distance of at least n/P of alength of an array of marking elements, n being an integer larger than1, P being a number of mutually interstitial printing steps used whenprinting an image.

[0014] According to a further aspect of the present invention, a firstactive marking element of the first array of marking elements is spacedapart in the direction of the longitudinal axis from a first activemarking element of the second array of marking elements over a distanceof at least n/I of a length of an array of marking elements instead ofn/P, n being an integer larger than 1, I being a number of interlacingprinting steps used when printing an image.

[0015] In yet a further aspect of the present invention, a printerprovided with a print head as described in a previous aspect of thepresent invention is described. Such a printer may be an ink jetprinter, for example a bubble jet printer.

[0016] The present invention includes a method of controlling a printhead, the print head comprising at least a first array of markingelements and a second array of marking elements for printing differentcolours, each array of marking elements comprising a set of equallyspaced marking elements, the method comprising the step of: inhibitingthe operation of second array marking elements not to be used, so thatthe first active marking element of the second array is spaced apartfrom the first active marking element of the first array.

[0017] The present invention also includes a control unit forcontrolling a print head, the print head comprising at least a firstarray of marking elements and a second array of marking elements forprinting different colours, each array of marking elements comprising aset of equally spaced marking elements, the control unit comprisingmeans for inhibiting the operation of second array marking elements notto be used, so that the first active marking element of the second arrayis spaced apart from the first active marking element of the firstarray.

[0018] The present invention also includes a computer program productfor executing any of the methods according to the present invention whenexecuted on a computing device associated with a printing head. Thepresent invention also includes a machine readable data storage devicestoring the computer program product. The present invention includestransmission of the computer product over a local or wide areatelecommunications network.

[0019] The present invention will now be described with reference to thefollowing drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 shows a printing head that may be used according to thepresent invention.

[0021]FIG. 2 illustrates mutually interstitial or mutually interspersedprinting.

[0022]FIG. 3 illustrates interlacing.

[0023]FIG. 4 illustrates printing of an image comprising a plurality ofsub-images according to the present invention, the printing comprisinginterlacing steps and mutually interstitial printing steps.

[0024]FIG. 5 shows a printed image consisting of different swaths.

[0025]FIG. 6 illustrates an implementation of software stagger.

[0026]FIG. 7 is a highly schematic representation of a inkjet printerfor use with the present invention.

[0027]FIG. 8 is a schematic representation of a printer controller inaccordance with an embodiment of the present invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0028] The present invention will be described with reference to certainembodiments and drawings but the present invention is not limitedthereto but only by the claims. The present invention will be describedwith reference mainly to ink-jet printing but the present invention isnot limited thereto. The term “printing” as used in this inventionshould be construed broadly. It relates to forming markings whether byink or other materials or methods onto a printing substrate. Variousprinting methods which may be used with the present invention aredescribed in the book “Principles of non-impact printing”, J. L.Johnson, Palatino Press, Irvine, 1998, e.g. thermal transfer printing,thermal dye transfer printing, deflected ink jet printing, ionprojection printing, field control printing, impulse ink jet printing,drop-on-demand ink jet printing, continuous ink jet printing.Non-contact printing methods are particularly preferred. However, thepresent invention is not limited thereto. Any form of printing includingdots or droplets on a substrate is included within the scope of thepresent invention, e.g. piezoelectric printing heads may be used toprint polymer materials as used and described by Plastic Logic(http://plasticlogic.com/) for the printing of thin film transistors.Hence, the term “printing” in accordance with the present invention notonly includes marking with conventional staining inks but also theformation of printed structures or areas of different characteristics ona substrate. On example is the printing of water repellent or waterattractive regions on a substrate in order to form an off-set printingplate by printing. Accordingly, the term “printing medium” or “printingsubstrate” should also be given a wide meaning including not only paper,transparent sheets, textiles but also flat plates or curved plates whichmay be included in or be part of a printing press. In addition theprinting may be carried out at room temperature or at elevatedtemperature, e.g. to print a hot-melt adhesive the printing head may beheated above the melting temperature. Accordingly, the term “ink” shouldalso be interpreted broadly including not only conventional inks butalso solid materials such as polymers which may be printed in solutionor by lowering their viscosity at high temperatures as well as materialswhich provide some characteristic to a printed substrate such asinformation defined by a structure on the surface of the printingsubstrate, water repellance, or binding molecules such as DNA which arespotted onto microarrays. As solvents both water and organic solventsmay be used. Inks as used with the present invention may include avariety of additives such as ant-oxidants, pigments and cross-linkingagents.

[0029] A dot matrix printing head of a kind which may be used with thepresent invention is shown schematically in FIG. 1. As shown in FIG. 1 ascanning printing head 10 may have an elongate form having alongitudinal axis 50. The printing head 10 comprises a plurality ofmarker elements 11, for example a plurality of ink jetting orifices 12-1. . . 12-n, 13-1 . . . 13-n, 14-1 . . . 14-n, 15-1 . . . 15-n for thecolours yellow, magenta, cyan and black each arranged in an array 12,13, 14, 15 respectively which may comprise one or more rows. As shown inFIG. 1 there are two rows 16, 17 per colour whereby the second row 17 isoffset by half a nozzle pitch np with respect to the first row 16.

[0030] Generally, the head 10 is moved relative to a printing medium(such as paper) in the direction indicated with the arrow “Y” known asthe fast scan direction which is, in the example given, perpendicular tothe longitudinal axis 50 of the head 10. In an alternative embodiment,not shown in the drawings, the head 10 may be placed in a slantedposition with regard to the fast scan direction, to increase theprinting resolution. The printing head 10 may comprise an ink cartridgecarried on a movable carriage assembly. By repeatedly firing the arrays12, 13, 14, 15 of nozzles 11 and moving in the fast scan direction inkdrops are deposited on the printing medium in parallel lines across theprinting medium in accordance with an image to be printed. Each line ofprinting from a single nozzle 11 is known as a raster line. When thehead 10 has traversed the printing medium it returns to its startingposition and the process begins again. The printing head 10 may print onthe way back—i.e. printing a second pass, or the printing head 10 mayonly print when moving in one direction. The printing medium may beindexed in the slow scan direction X (perpendicular to the fast scandirection Y and, in the example given, parallel to the longitudinal axis50 of the printing head 10) between passes. The firing of the nozzles 11is controlled by a control device, e.g. a microprocessor ormicrocontroller (see FIG. 7), the firing being in accordance with adigital representation of an image which is processed by the controldevice. The digital representation of an image may be provided by agraphics software program running on a host computer or by scanning inan image. In this way a complete image is printed.

[0031] Within an array of nozzles 12, 13, 14, 15 adjacent nozzles in theslow scan direction, e.g. 12-2, 12-4 have a spacing “np” (nozzle pitch).This is usually constant for an array.

[0032] First the concept of mutually interstitial printing or mutuallyinterspersed printing will be explained as applied to a traversing orscanning head 10 for printing one colour only (e.g. a black head). FIG.2 shows how an image is divided in sub-images, which are mutuallyinterstitially printed using a Mutual Interstitial Printing Ratio (MIPR)of 25% but which are not interlaced. When looking at FIG. 2 it wouldappear that the head 10 is displaced in a slow scan direction −X withrespect to the printing medium. This in fact refers to relative motionbetween the two and the typical implementation is that the printingmedium is transported a distance relative to the head 10, e.g. a quarterof a head length, in the opposite direction to that shown in FIG. 2(i.e. in the +X direction). In the following, it is preferred to referto the transport of the head 10 because the pixel position on theprinting medium is the reference.

[0033] In a first pass, nozzles in a first fraction, e.g. a firstquarter of the head 10 print every so many pixels, e.g. every fourthpixel in a column in the fast scan direction Y, beginning with the firstrow which is able to print. This is indicated by a 1 in the table ofFIG. 2. This means that the head 10 is transported relative to theprinting medium by an exact fraction of the head length, e.g. an exactnumber of nozzle pitches between the firing positions of the relevantnozzles. Note that whether or not the nozzles actually print depends onthe image to be printed, i.e. whether or not a dot is to be printed at acertain location. Thus, a 1 in the table indicates the ability of therelevant nozzle to print at a location—it does not mean that it alwaysprints at this location. Also, going down a “column” of the tables inthe attached figures refers to going along the fast scan direction Y,i.e. in the example given, the direction perpendicular to thelongitudinal axis 50 of the printing head 10.

[0034] After the first scan across the printing medium is complete, thehead 10 is returned to the starting position and is transported aquarter of its length with respect to the printing medium in the slowscan direction (X) ready for pass 2. With “length of the head” is meantthe length of the number of active nozzles available for the printingprocess. This is not necessarily the same as the length of the totalnumber of nozzles on the head as the present invention includes using asub-set of these nozzles for the printing operation. In this embodimentit is assumed that the head 10 does not print on the return trip butprinting in both fast scan directions Y and −Y is included within thescope of the present invention. In the second pass the first half of thehead 10 is printing every fourth pixel, beginning with the second row inthe table (indicated by a 2 in the table). After the second pass iscomplete the print head 10 is displaced a quarter of its length again.In the third pass the first ¾ of the head 10 is printing every fourthpixel, beginning with the third row (indicated by 3). The print head 10is transported a quarter of its length again in the slow scan direction.From now on the printer is printing with all nozzles every fourth pixel.The print head 10 is transported a quarter of its length again and thefifth pass (number 5) is printed every fourth row beginning with row 1again in a new cycle. Such cycles are repeated continuously.

[0035] The result of this is that a dot in a column (i.e. in the fastscan direction Y) is only printed with the same nozzle every fourpixels. Each adjacent dot in the Y direction is printed by a differentnozzle. This means that if one nozzle 12-1, 13-1, 14-1, 15-1 produces adefective dot, this defect is camouflaged to some extent by being mixedin with dots produced by nozzles 12-2, 12-3, 12-4; 13-2, 13-3, 13-4;14-2, 14-3, 14-4; 15-2, 15-3, 15-4, respectively.

[0036] The cycle repeats every four passes—this is 25% mutuallyinterstitial printing. Because in each column each four successive dotsare each printed with a different nozzle, banding problems due tonozzle-misalignment are hidden. The first passes don't have to have thesame length. They can have any length provided the following conditionis fulfilled: the distance represented by the sum of the head/printingmedium relative movements in the first P passes, where P is an integer(4 in the above example), has to be equal to the exact active nozzlelength, i.e. the length of the nozzle array of active nozzles (nozzleswhich are used to print or not print at a location), measured innozzles. So it is for example also possible to print 1, transport thehead a distance of a nozzle pitches, print 2; transport the head adistance of b nozzle pitches, print 3; transport the head a distance ofc nozzle pitches, print 4; transport the head a distance of e=n−(a+b+c)nozzle pitches, where e has to be larger than 0 and n equals the numberof active nozzles in the array. From this point on in the method thispattern has to be repeated until the complete image is printed.

[0037] Interlacing is a technique to obtain a higher resolution printedimage than would be expected based on the nozzle distance np. Forexample, interlacing allows writing a 720 dpi (dots per inch) image witha 180 dpi head (i.e. the nozzles are spaced on the head so as togenerate 180 dpi). With interlacing using a scanning head 10 the slowscan pixel pitch, that is the pitch of dots printed on the printingmedium in the slow scan direction is smaller than the nozzle pitch np ofthe head 10 in the slow scan direction. The slow scan direction for ascanning head 10 is perpendicular to the fast scan direction, and, inthe example given, parallel to the longitudinal axis 50 of the head.

[0038] To continue with the example above and referring to FIG. 3, toachieve a higher resolution a first part of the head 10 (e.g. the firstquarter) prints first every so many columns, e.g. every fourth column.Then the head 10 is transported by one pixel pitch+(k₁*nozzle pitch),(note: k₁ is an integer which may be zero). Then in the next pass thehead 10 prints again every so many columns, e.g. every fourth columnbeginning with the second one, then the print head is transported onepixel pitch+(k₂*nozzle pitch), (k₂ is an integer which may be zero).This procedure is repeated a number of times, e.g. a third time and afourth time, after which the print head can be displaced the rest of thehead length. The value of k (generally, k_(i)) can be chosen freely,e.g. in such a way that it is equal for every transport stepk₁=k₂=k₃=k₄.

[0039] In accordance with embodiments of the present invention bothmutually interstitial printing and interlacing are carried out toimprove printing quality and to avoid banding. For example, inaccordance with a first embodiment an image is divided in sub-imageswhich are mutually interstitially printed 25% (generally: 100/P % with Pthe number of passes for the mutually interstitial printing) andinterlaced to order 4. The image-resolution is 720 dpi, therefore it hasto be written in 4 swaths using a head with a nozzle pitch of 140 μm(180 dpi). So, to write the complete image the head has to make 4 (P)(due to the mutually interstitial printing)×4 (I) (due tointerlacing)=16 passes. This printing method will be described withreference to FIG. 4, for example for a 720 dpi image formed whensub-images are mutually interstitially printed 25% and interlaced toorder 4.

[0040] It will be assumed for this embodiment of the present inventionthat mutually interstitial printing is done at 25% and the interlacingis of order 4, that is the pixel pitch in the slow scan direction is onequarter of the nozzle pitch np in the same direction on the head 10. Theresult is an image made up of 16 sub-images, each sub-image having aresolution of 180 dpi in the slow/fast scan direction. The presentinvention is not limited to the same number of mutually interstitialprinting passes as interlacing passes, each number can be chosen freelyprovided the interlacing order is at least 2 and the mutuallyinterstitial printing as 50% or less. In addition only one colour willbe considered when describing the present case although the inventionmay be applied to the coloured printing case as will be described below.

[0041] Referring to FIG. 4 the head 10 writes first an image made up ofthe pixel positions having the “11” symbol. In the reference digit 11the first digit “1” is the number of the pass used in mutuallyinterstitial printing, the second digit “1” is the number of the pass ininterlacing. An example of the printing procedure is given below:

[0042] The head 10 writes during the first pass with nozzle 1 in column1 at pixels defined by symbol “11” in rows 1, 5, 9 etc. along the fastscan direction. The head 10 prints with nozzle 2 in the same pass pixelpositions defined by the symbol “11” in column 5, row 1, 5, 9 etc. andprints with nozzle 3 the pixels defined by the symbol “11” in column 9,row 1, 5, 9 etc. The same is happening for the other nozzles. Afterprinting the complete sub-image for “11”, the head writes, during asecond pass, the sub-image defined by the symbol “12” in the same way.Hence, after the first (“11”) sub-image the head moves a pixel pitchplus k times a nozzle distance and the first interlacing level isperformed for all mutually interstitial printing operations to completeanother sub-image (e.g. “12”). Any other of the symbols can also beprinted in this fashion. These 16 sub-images can be written completelyindependent from each other. Therefore, in general, the interlacingsteps will be intercalated with mutually interstitial printing so thatall the sub-images are being created concurrently rather than one afteranother. In fact the order in which the sub-images are printed, i.e. theway the printing traverses through the sub-image matrix

11, 12, 13, 14

21, 22, 23, 24

31, 32, 33, 34

41, 42, 43, 44  (1)

[0043] is freely selectable. The only requirement is that each one ofthe positions in selected once.

[0044] A swath is defined by a part of a sub-image printed within a headlength in one pass as shown in FIG. 5, the head length being the lengthof active nozzles able to fire in one pass. Thus, as the printing of onepass of one sub-image (e.g. the “11” sub-image) is completed whosedistance in the slow scan direction X is one head length (the length ofhe active nozzles for printing), one swath has been printed. Then thesecond swath for the “11” sub-image is printed. In reality the secondswath for one symbol, e.g. for “11”, will typically be printed afterthat all the first swaths of the other sub-images are printed as theprinting head makes many passes as it slowly progresses in the slow scandirection. This means that the printing goes through the sixteenpositions of the above matrix before returning to a second swath for the“11” symbol. That is, swath 2 of symbol “11”, is actually the 17^(th)pass in an image with 16 sub-images. All the other first swaths of eachsub-image are printed first before the second swath of the first symbolis printed.

[0045] Generally, the number of sub-images in one image (N) is theproduct of the number of mutually interstitial printing passes P and thenumber of interlacing passes I

N=P*I  (2)

[0046] Because a colour image is composed of a number of differentcolour separations, e.g. typically 3 or 4 different monochrome images,each colour separation will be printed using the same independentcombination of 16 swaths. So, a full colour image which is mutuallyinterstitially printed 25% and has a resolution 720 dpi can be writtenwith a head of 180 dpi in 4×16 sub-images=64 independent sub-images.Generally, the number of sub-images (or swaths) is given by:

N=P*I*C  (3)

[0047] where C=Number of colours.

[0048] This can be presented as a cube with on each level one of thesquare matrices as explained above and on each column of the cube acolour.

[0049] It is preferred in accordance with an embodiment of the presentinvention if the swath boundaries or swath transition lines for thesub-images “11” etc. do not fall together on the same line. Ideally, notwo swath boundaries fall together onto one line. By distributing theswath boundaries through the printing, any systematic error caused bythe length of the printing head can be hidden. It is the selection ofthe sequence of traversing the sub-image matrix (see (1) above) whichdetermines where the swath boundaries will lie.

[0050] In accordance with further embodiments of the present inventionbanding due to paper transport can be suppressed. To achieve this thestep distance for the relative motion between printing medium and theprint head in the printing direction needs to be controlled.

[0051] To avoid banding due to transport of the printing medium, it isnecessary to write images in such a way, that the swath transition linesfor every sub-image is on a different place. This can be achieved if theprinting is carried out in accordance with the next equations. Thisprocedure will achieve that the swath transition lines are homogeneouslyspread over the image.

[0052] Firstly, the number of transport steps (=T) to reach one headlength is given by:

T=N/h=(C×P×I)/h  (4)

[0053] where h is the number of nozzle rows written at the same time.This equation defines the number of transport steps T in one head lengthto be the number of sub-images divided by the number of rows which areprinted at the same time. If the swath transition lines are to be spreadequally over this distance then the transport distance step TD isdefined approximately by:

TD=n/T  (5)

[0054] n being the number of nozzles in one nozzle row. These transportsteps are preferably performed in at least 2 different step lengths inorder to reach every position of the image.

[0055] For example: for I=4; and all I are written with the same head,the distances moved are:

n/T−1dp/np

n/T−1dp/np

n/T−1dp/np

n/T+3dp/np

[0056] where dp is the pixel pitch and np is the nozzle pitch. Thissequence of movements is repeated [(C×P)/h] times in order to completeone head length of the image.

[0057] Mutually interstitial printing of sub-images is used in the aboveexamples to avoid banding due to nozzle misalignment. It is generallyheld that it is not possible to mutually interstitially print withoutslowing down the throughput of the system or without making asignificant number of the nozzles in a head idle some of the time. Inaccordance with an embodiment of the present invention mutuallyinterstitial printing of sub-images can be made more efficient byincreasing the speed of traverse in the fast scan direction. Becauseeach sub-image is a 180 dpi image, and because each of these images isindependent of each other, each sub-image can be written with a minimumtime between two neighbouring pixels. This is called fast mutuallyinterstitial printing. This means the first row and the second row of asub-image (the second row in a 25% mutually interstitially printed imageis the 5^(th) row of the image as there are pixels from three othersub-images in between), can be printed after the shortest time possiblebetween two dots for example, 100 μs if a 10 kHz head is used, while inconventional mutually interstitial printing, there are 100 μs betweeneach two lines of the image to be printed, thus for 25% mutuallyinterstitial printing with a 10 kHz head there is 400 μs between thefirst and the second row of a sub-image. None of the intermediate pixelshave to be printed in the same time when using fast mutuallyinterstitial printing. Accordingly, all the active nozzles of theprinting head relevant to one colour can be available for printing ateach relevant position, e.g. “11” or similar. This means that the headis used to optimum efficiency by combining interlacing and mutuallyinterstitial printing. However, the present invention also includes asingle printing operation of a line of dots with less than the fullcompliment of active nozzles, i.e. to select a specific redundancy ofthe nozzles, for example only every other active nozzle is available forfiring at each print operation. This is the same as conventionalmutually interstitial printing of sub-images in which there is aredundancy in the number of nozzles. If every other nozzle is used inone pass, this would mean a redundancy of 50%. To print with the othernozzles a further pass is required. The effect of the redundancy is thata sub-image is divided further into more sub-images, however, thisprinting is still mutual interstitial printing of all these sub-imagesin accordance with the present invention.

[0058] Furthermore, it is possible according to the present invention touse mixed mutually interstitial printing, which is a combination of fastand normal mutually interstitial printing. This means that part of animage is printed by fast mutually interstitial printing, and the otherpart of the image is printed by normal mutually interstitial printing.In this way, redundancy is obtained: one pixel can be reached more thanonce.

[0059] Preferably, the fast mutually interstitial printed part of theimage comprises the highest possible number of sub-images, preferablyall sub-images. However, any combination of fast and slow mutuallyinterstitial printing is possible, e.g. one sub-image being fastmutually interstitial printed, and all the other sub-images beingcoventionally mutually interstitially printed.

[0060] The printing head 10 shown in FIG. 1 illustrates a printing head10 consisting of four heads 22, 23, 24, 25, for yellow, cyan, magenta,and black respectively. Each head has a plurality of ink jettingorifices 12-1 . . . 12-n, 13-1 . . . 13-n, 14-1 . . . 14-n, 15-1 . . .15-n for each colour.

[0061] To avoid banding caused by non-homogeneously spreading of theswath transition line, the distance in the slow scan direction X betweenthe first nozzle 12-2, 13-2, 14-2, 15-2 of a first nozzle row 16 of ahead 22, 23, 24, 25, and the first nozzle 12-1, 13-1, 14-1, 15-1 of asecond nozzle row 17 of that head 22, 23, 24, 25, further called x1, andthe distance in the slow scan direction X between the first nozzle 12-2,13-2, 14-2 of a first nozzle row 16 of a first head 22, 23, 24 and thefirst nozzle 13-2, 14-2, 15-2 of a first nozzle row 16 of a second head23, 24, 25, further called x2, should be chosen such that all heads areequally spread over the distance TD. This can be done by choosing x1 andx2 equal to approximately TD/h. In this way all swath transition lineswill be spread equally over the image. The configuration of the headscan be optimised in this way.

[0062] The most interesting distance of the heads with respect to eachother will be as follows:

x 2=[integer (TD/h)+i*0.25+k*TD]np

[0063] with k an integer, and i=0, 1, 2, 3. To have a faster throughput,k should be as low as possible, preferably k=0.

[0064] It is also possible to spread the heads unequally over thedistance TD. This, however, has the disadvantage that the sub-imageseparation lines are not spread as homogeneously as possible over theimage.

[0065] For heads with two nozzle rows 16, 17, as represented in FIG. 1,the same formula is also valid for x1, but now i=2 and k=0:

x 1=[integer (TD/h)+0.5]np

[0066] The distance in the fast scan direction Y between the firstnozzle 12-2, 13-2, 14-2, 15-2 of a first nozzle row 16 of a head 22, 23,24, 25 and the first nozzle 12-1, 13-1, 14-1, 15-1 of a second nozzlerow 17 of that head 22, 23, 24, 25, further called y1, and the distancein the fast scan direction Y between the first nozzle 12-2, 13-2, 14-2of a first nozzle row 16 of a first head 22, 23, 24 and the first nozzle13-2, 14-2, 15-2 of a first nozzle row 16 of a second head 23, 24, 25,further called y2, are not important because the place where a dot isprinted depends on the time when it is printed. Any pixel position isreachable, just by changing the moment of firing.

[0067] For example, with a 180 dpi head a full colour image of 720 dpiis to be printed. Therefore the image is to be divided in sub-imageswhich are mutually interstitially printed 25% and interlaced 4 times.Four heads 22, 23, 24, 25 are used, each having two nozzle rows 16, 17each comprising n nozzles. Each head 22, 23, 24, 25 has a differentcolour, e.g. yellow, magenta, cyan, black. From the above, it is easy tocalculate that his image consists of 64 sub-images (C*P*I). Each ofthese sub-images can be written by a head that writes the first ncolumns. The paper is transported, and the following n columns arewritten, etc., as explained with regard to FIG. 2. As there are, in thepresent example, 4 heads 22, 23, 24, 25 each having two nozzle rows 16,17, eight swaths are printed in the same time before a next papertransport.

[0068] In order to avoid that the sub-image separation lines are on thesame place, the heads 22, 23, 24, 25 should be organised so that thedistance between nozzles of different nozzle rows is as indicatedhereinabove. For the present system this means:

[0069] h (number of nozzle rows)=8 (4 heads with each 2 nozzle rows)

[0070] n (number of nozzles in a row)=382

[0071] C (number of colours)=4

[0072] P (number of mutually interstitial printing passes)=4

[0073] I (number of interlacing passes)=4

[0074] Therefore N (number of sub-images)=C*P*I=64

[0075] If the following formulae are used:

[0076] T=N/h

T=64/8=8

[0077] TD=n/T and a rest

TD=382/8=47 rest 6

[0078] x2=[integer (TD/h)+i*0.25+k*TD]np, with k an integer and i=(0, 1,2, 3), and i=2 and k=8 is chosen (the following head is chosen to beplaced without overlap with the first head)

[0079] x1=[integer (TD/h)+0.5]np,

[0080] then:

[0081] x2=[integer (47/8)+2*0.25+8*47]np=381.5 np=381.5*141.11 μm=53834μm

[0082] x1=[integer (47/8)+0.5]np=5.5*141.11 μm=776 μm

[0083] So the first nozzle 12-2, 12-1, 13-2, 13-1, 14-2, 14-1, 15-2,15-1 of each nozzle row 16, 17 is as shown in the following table inreference to the first nozzle of the first head: Nozzle row 16 Nozzlerow 16 head 22 0 μm 776 μm head 23 54610 μm 55386 μm head 24 109220 μm109996 μm head 25 163830 μm 164606 μm

[0084] When the heads are arranged as in the table, and TD=alternating46.75 and 47.25 np (so that the average is 47 np), and this process isrepeated 8 times (once for each swath), all sub-image separation linesare equally spread over the image.

[0085] The image process steps look as follows: when the first nozzlearray of the first head is writing the “11”-sub-image, the second nozzlearray of the same head can write one of the following images: “13”,“23”, “33”, “43”. Which of those images can be printed, depends on thetiming when the dots are printed. The other 15 locations are alsopossible, but then another configuration of the heads is necessary.

[0086] It is important that no neighbours are printed during a samepass, in order to avoid banding and coalescence. Therefore, if the firstnozzle row is writing “11”, it is best to choose for the second nozzlerow a pixel position in the cross area indicated in the following table:

[0087] To write a complete image, all 16 sub-images of a same colourhave to be printed. Therefore the first nozzle row of a head is writing8 sub-images, and the second nozzle row of that head is writing theother 8 sub-images. A possible combination can be found in the followinglist:

[0088] When nozzle row 1 is writing 11 nozzle row 2 is writing 13

[0089] When nozzle row 1 is writing 12 nozzle row 2 is writing 14

[0090] When nozzle row 1 is writing 21 nozzle row 2 is writing 23

[0091] When nozzle row 1 is writing 22 nozzle row 2 is writing 24

[0092] When nozzle row 1 is writing 31 nozzle row 2 is writing 33

[0093] When nozzle row 1 is writing 32 nozzle row 2 is writing 34

[0094] When nozzle row 1 is writing 41 nozzle row 2 is writing 43

[0095] When nozzle row 1 is writing 42 nozzle row 2 is writing 44

[0096] In this way bleeding is not possible, because the dots which areprinted during a same pass have a distance of 71 μm and a dot is notthat big.

[0097] It is to be noted that TD=integer (382/8)+rest=47+6. Becauseafter T transport steps the head is displaced T*TD=8*47=376 np, the last6 nozzles of the head cannot be used. Indeed, after those T*TD transportsteps, the head has to print the next swath for each sub-image.

[0098] Because colours have to be printed in a certain order (e.g. firstyellow, then magenta, then cyan, then black), a number of passes shouldoccur in order to fulfil the following conditions:

[0099] a dot of a second colour can only be printed upon a pixelposition after the first colour has been printed there,

[0100] to avoid bleeding, no two pixels which are each others neighbourscan be printed at the same time,

[0101] a dot of a second colour can only be printed if all itsneighbours are printed in the first colour to avoid colour differencescaused by different colour overlap.

[0102] To obtain this, nozzle rows printing different colours arestaggered.

[0103] If two nozzle rows are used to write two different colours, thedistance x2 between the first nozzle of the first nozzle row and thefirst nozzle of the second nozzle row has to be at least (2*I/hs)/(T) ofa nozzle row length in the slow scan direction, with I the number ofinterlacing passes, hs the number of nozzle rows printing the samecolour, and T the number of transport steps to reach one head length.When overlapping dots still deteriorate the image result, the distancex2 has to be at least (3*I/hs)/(T) times the nozzle row length in theslow scan direction. In this case a drop of the second colour is alwayson top of a drop of the first colour.

[0104] The higher the number of mutually interstitial printing passes Pis chosen, the nearer the nozzle rows can be put to each other in theslow scan direction. This is because(3*I/hs)/T=(3*I/hs)/(N/h)=(3*I*(h/hs))/N=(3*I*C)/(P*I*C)=3/P. Therefor,in order to fulfil the conditions for colour order, the maximum overlapfor heads printing two different colours is 1−3/P. In the same way, itcan be calculated that in order to fulfil the conditions for colourorder, the maximum overlap for heads printing two different colours willbe 1−3/I. As mutually interstitial printing is more flexible thaninterlacing, the preferred overlap of the heads is 1−3/P.

[0105] Instead of physically staggering the rows printing differentcolours, also a stagger can be implemented, as schematically shown inFIG. 6 by influencing the operation of nozzles, e.g. by providingsoftware to drive a printing head in such a way as to provide thestagger in active nozzles. A print head 60 with a longitudinal axis 50is provided with at least two arrays 62, 72 of nozzles for printing twodifferent colours. Both arrays 62, 72 of nozzles comprise the sameamount of nozzles, with a same nozzle pitch, and a first nozzle of thearray 62 is located on a same position in the slow scan direction as afirst nozzle of the array 72.

[0106] When using software stagger, not all the nozzles in each array62, 72 of nozzles are used. A first selection of nozzles of array 62 ismade, thus generating a first set of nozzles 64, and a second selectionof nozzles of array 72 is made, thus generating a second set of nozzles74. Only the nozzles of the first set 64 and the second set 74 are usedwhen printing an image, they can be fired or not depending on the imagecontent. The other nozzles, not being part of the first set 64 or thesecond set 74, are not used at all, or may be used as replacementnozzles for replacing a defective nozzle.

[0107] The choice of the first set 64 and the second set 74 is such thata first nozzle of the first set 64 of nozzles is spaced apart in thedirection of the longitudinal axis 50 from a first marking element ofthe second set 74 of nozzles over a distance d of at least 2/P,preferably 3/P, of a length of the array 62 of nozzles, P being a numberof mutually interstitial printing steps used when printing an image.Alternatively, the choice of the first set 64 and the second set 74 issuch that a first nozzle of the first set 64 of nozzles is spaced apartin the direction of the longitudinal axis 50 from a first markingelement of the second set 74 of nozzles over a distance d of at least2/I, preferably 3/I, of a length of the array 62 of nozzles, I being anumber of interlacing printing steps is used when printing an image.

[0108] Software stagger makes printing slower than using all nozzles ofeach head 60, 70, because some of the nozzles are filtered out to inorder to continuously block or deactivate them. However, it is a way ofvirtually increasing the lifetime of the print head 60, 70.

[0109] In the next table, the results are described for differentnumbers of mutually interstitial printing passes: Nozzle row Nozzle rowoverlap overlap (all Mutually Passe (not all neighbours neighbours areinterstitia s are printed) printed) 1 printing P 1-2/P 1-3/P 50% 2 0 025% 4 2/4 = 1/2 1/4   12.5% 8 6/8 = 1/2 5/8

[0110]FIG. 7 is a highly schematic general perspective view of an inkjetprinter 20 which can be used with the present invention. The printer 20includes a base 31, a carriage assembly 32, a step motor 33, a drivebelt 34 driven by the step motor 33, and a guide rail assembly 36 forthe carriage assembly 32. Mounted on the carriage assembly 32 is a printhead 10 that has a plurality of nozzles. The print head 10 may alsoinclude one or more ink cartridges or any suitable ink supply system. Asheet of paper 37 is fed in the slow scan direction over a support 38 bya feed mechanism (not shown). The carriage assembly 32 is moved alongthe guide rail assembly 36 by the action of the drive belt 34 driven bythe step motor 33 in the fast scanning direction.

[0111]FIG. 8 is a block diagram of the electronic control system of aprinter 20, which is one example of a control system for use with aprint head 10, 60 in accordance with the present invention. The printer20 includes a buffer memory 40 for receiving a print file in the form ofsignals from a host computer 30, an image buffer 42 for storing printingdata, and a printer controller 60 that controls the overall operation ofthe printer 20. Connected to the printer controller 60 are a fast scandriver 62 for a carriage assembly drive motor 66, a slow scan driver 64for a paper feed drive motor 68, and a head driver 44 for the print head10, 60. In addition there is a data store 70 for storing parameters forcontrolling the interlaced and/or mutual interstitial printing operationin accordance with the present invention. Host computer 30 may be anysuitable programmable computing device such as personal computer with aPentium III microprocessor supplied by Intel Corp. USA, for instance,with memory and a graphical interface such as Windows 98 as supplied byMicrosoft Corp. USA. The printer controller 60 may include a computingdevice, e.g. microprocessor, for instance it may be a microcontroller.In particular, it may include a programmable printer controller, forinstance a programmable digital logic element such as a ProgrammableArray Logic (PAL), a Programmable Logic Array, a Programmable GateArray, especially a Field Programmable Gate Array (FPGA). The use of anFPGA allows subsequent programming of the printer device, e.g. bydownloading the required settings of the FPGA. The printer controllercan carry out the inhibition of certain nozzles on the print head 60 inorder provide a stagger between rows as has been described above.

[0112] The user of printer 20 can optionally set values into the datastore 70 so as to modify the operation of the printer head 10. The usercan for instance set values into the data store 70 by means of a menuconsole 46 on the printer 20. Alternatively, these parameters may be setinto the data store 70 from host computer 30, e.g. by manual entry via akeyboard. For example, based on data specified and entered by the user,a printer driver (not shown) of the host computer 30 determines thevarious parameters that define the printing operations and transfersthese to the printer controller 60 for writing into the data store 70.One aspect of the present invention is that the printer controller 60controls the operation of printer head 10 in accordance with settableparameters stored in data store 70. Based on these parameters, theprinter controller reads the required information contained in theprinting data stored in the buffer memory 40 and sends control signalsto the drivers 62, 64 and 44.

[0113] For instance, the printing data is broken down into theindividual colour components to obtain image data in the form of a bitmap for each colour component which is stored in the receive buffermemory 30. The sub-images are derived from this bit map, in particulareach sub-image will start at a certain offset within the bit map. Inaccordance with control signals from the printer controller 60, the headdriver 44 reads out the colour component image data from the imagebuffer memory 52 in accordance with a specified sequence of printing thesub-images and uses the data to drive the array(s) of nozzles on theprint head 10 to mutually interstitially print the sub-images. The datawhich is stored in data store 70 may comprise one or more of:

[0114] a) the interlacing depth, i.e. the number interlaced lines ofprint

[0115] b) the redundancy of the mutual interstitial printing, that isthe percentage of the active print nozzles which are used at each lineprinting operation,

[0116] c) the number of passes which will make up the interstitialprinting operation, and

[0117] d) the offset in the bit map to be printed for each such pass.

[0118] The present invention includes the storing of alternativerepresentations of this data which however amount to the same techniqueof printing. In each case a) to d) there can be a default value which isassumed to apply if the user does not enter any values. Also, inaccordance with the present invention at least one of the parameters a)to d) is settable by the user. With respect to d), the sequence ofoffsets (and therefore the sequence of dealing with the sub-images) can,for instance, in one embodiment be freely specified by the user andthere can be a default sequence if the user does not specify a sequence.This ability to set the sequence allows the user to choose the order inwhich the sub-images are printed. It will also be appreciated from theabove that the user may freely set the number of sub-images to beprinted by selecting one or more of the number of passes, the percentageredundancy and the number of interlacing lines. hence, the user mayselect the complexity of the printing process which has an effect on thequality of print (e.g. lack of banding effects, masking defectivenozzles) as well as the time to print (number of passes before theprinting is complete). The controller 60 can use the number of passes ofthe printing operation to calculate the stagger required for head 60 andto inhibit the required nozzles on the head.

[0119] The present invention also includes that items a) to d) above aremachine settable, for instance printer controller 60 sets the parametersfor printing, e.g. at least one of items a) to d) above, e.g. inaccordance with an optimised algorithm. As indicated above thecontroller 60 may be programmable, e.g. it may include a microprocessoror an FPGA. In accordance with embodiments of the present invention aprinter in accordance with the present invention may be programmed toprovide different levels of printing complexity. For example, the basicmodel of the printer may provide selection of at least one of the numberand sequence of printing of the sub-images. An upgrade in the form of aprogram to download into the microprocessor or FPGA of the controller 60may provide additional selection functionality, e.g. at least one of thedegree of interlacing and the nozzle redundancy. Accordingly, thepresent invention includes a computer program product which provides thefunctionality of any of the methods according to the present inventionwhen executed on a computing device. Further, the present inventionincludes a data carrier such as a CD-ROM or a diskette which stores thecomputer product in a machine readable form and which executes at leastone of the methods of the invention when executed on a computing device.Nowadays, such software is often offered on the Internet or a companyIntranet for download, hence the present invention includes transmittingthe printing computer product according to the present invention over alocal or wide area network. The computing device may include one of amicroprocessor and an FPGA.

[0120] The data store 70 may comprise any suitable device for storingdigital data as known to the skilled person, e.g. a register or set ofregisters, a memory device such as RAM, EPROM or solid state memory.

[0121] While the invention has been shown and described with referenceto a preferred embodiment, it will be understood by those skilled in theart that various changes or modifications in form and detail may be madewithout departing from the scope and spirit of this invention. Forinstance, with reference to FIG. 8 the parameters for determining thecombined mutual interstitial and interlaced printing are stored in datastore 70. However, in accordance with the present invention thepreparation for the printing file to carry out the above mentionedprinted embodiments may be prepared by the host computer 30 and theprinter 20 simply prints in accordance with this file as a slave deviceof the host computer 30. Hence, the present invention includes that theprinting schemes of the present invention are implemented in software ona host computer and printed on a printer which carries out theinstructions from the host computer without amendment. Accordingly, thepresent invention includes a computer program product which provides thefunctionality of any of the methods according to the present inventionwhen executed on a computing device which is associated with a printinghead, that is the printing head and the programmable computing devicemay be included with the printer or the programmable device may be acomputer or computer system, e.g. a Local Area Network connected to aprinter. The printer may be a network printer. Further, the presentinvention includes a data carrier such as a CD-ROM or a diskette whichstores the computer product in a machine readable form and which canexecute at least one of the methods of the invention when the programstored on the data carrier is executed on a computing device. Thecomputing device may include a personal computer or a work station.Nowadays, such software is often offered on the Internet or a companyIntranet for download, hence the present invention includes transmittingthe printing computer product according to the present invention over alocal or wide area network.

1. An elongate print head with a longitudinal axis, comprising at leasta first array of marking elements and a second array of marking elementsfor printing different colours, each array of marking elementscomprising a set of equally spaced marking elements, a first activemarking element of the first array of marking elements being spacedapart in the direction of the longitudinal axis from a first activemarking element of the second array of marking elements over a distanceof at least n/P of a length of an array of marking elements, n being aninteger larger than 1, P being a number of mutually interstitialprinting steps used when printing an image.
 2. A print head according toclaim 1, wherein the distance is at least 3/P.
 3. A print head accordingto claim 1, wherein at least two arrays of marking elements are locatedon the same head.
 4. A print head according to claim 1, furthercomprising a control unit for inhibiting the operation of second arraymarking elements not to be used, so that the first active markingelement of the second array is spaced apart from the first activemarking element of the first array.
 5. An elongate print head with alongitudinal axis, comprising at least a first array of marking elementsand a second array of marking elements for printing different colours,each array of marking elements comprising a set of equally spacedmarking elements, a first active marking element of the first array ofmarking elements being spaced apart in the direction of the longitudinalaxis from a first active marking element of the second array of markingelements over a distance of at least n/I of a length of an array ofmarking elements, n being an integer larger than 1, I being a number ofinterlacing steps used in printing an image.
 6. A print head accordingto claim 5, wherein the distance is at least 3/I.
 7. A print headaccording to claim 5, wherein at least two arrays of marking elementsare located on the same head.
 8. A print head according to claim 5,further comprising a control unit for inhibiting the operation of secondarray marking elements not to be used, so that the first active markingelement of the second array is spaced apart from the first activemarking element of the first array.
 9. A printer provided with a printhead according to claim
 1. 10. A printer according to claim 9, theprinter being an ink jet printer.
 11. A printer according to claim 10,the printer being a bubble jet printer.
 12. A printer provided with aprint head according to claim
 5. 13. A printer according to claim 12,the printer being an ink jet printer.
 14. A printer according to claim13, the printer being a bubble jet printer.
 15. A method of controllinga print head, the print head comprising at least a first array ofmarking elements and a second array of marking elements for printingdifferent colours, each array of marking elements comprising a set ofequally spaced marking elements, the method comprising the step of:inhibiting the operation of second array marking elements not to beused, so that the first active marking element of the second array isspaced apart from the first active marking element of the first array.16. The method according to claim 15, wherein the inhibition step isadapted so that the first active marking element of the first array ofmarking elements is spaced apart in the direction of the longitudinalaxis from a first active marking element of the second array of markingelements over a distance of at least n/P of a length of an array ofmarking elements, n being an integer larger than 1, P being a number ofmutually interstitial printing steps used when printing an image.
 17. Amethod according to claim 15, wherein the inhibition step is adapted sothat the first active marking element of the first array of markingelements is spaced apart in the direction of the longitudinal axis froma first active marking element of the second array of marking elementsover a distance of at least n/I of a length of an array of markingelements, n being an integer larger than 1, I being a number ofinterlacing steps used in printing an image.
 18. A control unit forcontrolling a print head, the print head comprising at least a firstarray of marking elements and a second array of marking elements forprinting different colours, each array of marking elements comprising aset of equally spaced marking elements, the control unit comprisingmeans for inhibiting the operation of second array marking elements notto be used, so that the first active marking element of the second arrayis spaced apart from the first active marking element of the firstarray.
 19. The control unit according to claim 18, wherein the means forinhibiting is adapted so that the first active marking element of thefirst array of marking elements is spaced apart in the direction of thelongitudinal axis from a first active marking element of the secondarray of marking elements over a distance of at least n/P of a length ofan array of marking elements, n being an integer larger than 1, P beinga number of mutually interstitial printing steps used when printing animage.
 20. The control unit according to claim 18, wherein the means forinhibiting is adapted so that the first active marking element of thefirst array of marking elements is spaced apart in the direction of thelongitudinal axis from a first active marking element of the secondarray of marking elements over a distance of at least n/I of a length ofan array of marking elements, n being an integer larger than 1, I beinga number of interlacing steps used in printing an image.
 21. A computerprogram product for executing the method as claimed in claims 15 whenexecuted on a computing device associated with a printing head.
 22. Amachine readable data storage device storing the computer programproduct of claim
 21. 23. Transmission of the computer product of claim21 over a local or wide area telecommunications network.